"Anti-Bactaria" Liquid for toys

Hi All,
Recently I repaired my son toy that was broken,
The toy contains a liquid with stuff floating on it.
The Liquid was leaking, so i had to seal it and refill.
I refilled it with TAP WATER and it worked fine. After few days the water was contaminated with something white that looked like a bacteria / fungi (something organic that was grown every day).
The water become so unclear so I have to remove them.
I can't fill it with solvent because this toy has a delicate plastic pump and valves and I'm afraid it will destroy it.

NOW AFTER THIS LONG STORY COME MY QUESTION -

What kind of liquid these toys are filled with? is there a DIY substitute.
by the way, the original liquid looked kind of oily.